Offset printing apparatus generally comprise a plate cylinder tangential to a blanket cylinder, with a supple superficial layer, which transfers a layer of ink onto a web of material to be printed advancing between the blanket cylinder and an impression cylinder applied under pressure against the former, this impression cylinder being able to be constituted by a blanket cylinder in the case of recto-verso printing on the web. In certain apparatus, the plate cylinder and the blanket cylinder are mounted between the two lateral cheeks of a removable cassette which may be displaced transversely with respect to the printing apparatus, i.e. perpendicularly to the direction of advance of the web to be printed.
In these offset printing apparatus incorporating a removable cassette, the various adjustments necessary for printing are obtained by wedging the cylinders either with the aid of cams or by the application of the cylinders on sloping shims. As a general rule, wedging between the plate cylinder and the blanket cylinder is effected by a displacement of the blanket cylinder; the skewing, i.e. the slight relative inclination of the axes of the plate and blanket cylinders, is effected by a displacement of the plate cylinder; and the wedging between the blanket cylinder and the impression cylinder (or other blanket cylinder) is effected by the displacement of the impression cylinder. With such a type of assembly, all the cylinders must have a possibility of displacement in order to be able to perform the functions of printing. Now, it is known that, in order to obtain high-quality printing, the blanket cylinder must be as rigid as possible and, to that end, cams must be produced, with a view to the displacement of the blanket cylinder, having clearances of some micrometers and most often with a supply of a lubricant under pressure in order to have relatively easy manoeuvre and minimum wear in time. This type of solution is therefore relatively expensive in order to have a correct printing function.
It is an object of the present invention to overcome this drawback by providing an offset printing apparatus of the type incorporating a removable cassette, of reduced overall cost due to a considerable simplification of its assembly.